The manufacture of integrated circuits (ICs) generally involves the use of complex lithographic processes to form microscopic solid-state devices and circuits in semiconductor wafers. These lithographic processes typically include forming layers of material on the wafer, patterning the layers, doping the substrate and/or the patterned layers and heat-treating (e.g., annealing) the resulting structures. These processes are then repeated to build up the IC structure. The result is a wafer containing a large number of ICs.
After the wafer is formed, it then will typically go through a sorting process. Sorting involves electrically testing each IC chip on the wafer for functionality. After sorting, the wafer is separated into individual IC chips, which are then packaged individually or in groups for incorporation onto a substrate, such as a printed circuit board (PCB).
At this stage of the process, the individual ICs are typically referred to as dies. These dies must then be placed on and fixed to specific locations on a substrate such that they become electronically and/or optically connected to other components with which they are designed to interact.
The machine responsible for placing the dies on a substrate is referred to as a die placement system, and sometimes as a “chip shooter”. Such a machine typically includes an optical vision system that locates and recognizes fiducials and other alignment marks or features on the die or elements attached thereto. Information from the optical vision system relating to the position of the die relative the alignment marks allows the die to be placed on the substrate at a specific location.
During such a die placement operation, die positioning must be very precise and accurate to ensure that interconnections between the die and substrate are properly established. To this end, substrates commonly include alignment marks or “fiducials” to assist in die placement. The placement of semiconductor devices onto a substrate or a printed circuit board is generally referred to in the industry as the die placement, die attach or die bonding operation.
Similarly, when placing dies, especially dies that can vary, sometimes considerably, in size and thickness, it is critical that they be placed with a consistent, precise and accurate amount of force. For example, when placing a die on epoxy, it is critical that, after placement, the epoxy bond line thickness criteria are met. When placing very thin dies (50 um or less) in a eutectic attach process (i.e. without the use of epoxy) the impact force, if high, can lead to die cracks. The margin of error in such placement operations is quite small.
Current methods of applying forces during a die placement, attach or bonding operation usually involve compressing a spring by a set distance. Disadvantages of a spring based system include: the additional displacement, as compared to alternative designs, needed to achieve the desired force, which results in additional cycle time and inaccuracy; overshoot in displacement that is difficult, if not impossible, to prevent with such systems results in the applied force varying; and attempts to minimize overshoot by lowering the speed at which the system operates decreases the machine's productivity.
Another known method of applying force during such an operation involves bringing the die placement head to a known calibrated/taught height. One major disadvantages of this method is that, in practice, there are variations in substrate heights and die thickness that result in inconsistent force being applied to the die and substrate.
In many instances, the challenge of such operations is increased due to the need for die placement systems to place multiple dies, which are often different from one another, on a single substrate. Since different types of dies are not uniform in size or shape, the die placement head, the portion of the die placement system in contact with the dies themselves and ultimately responsible for their precise placement, is specific to each type of die to be placed. In such situations, the die placement head must be swapped during the die placement operation with one that corresponds to the next die type to be placed before the next die placement operation can proceed.
This swapping of the die placement head can be manual or automatic. Manual methods require opening of the machine by a human operator and subsequent exchange of the die placement head while automatic swapping is typically accomplished by moving the die placement head to a tool carrier and swapping out the die placement head for one corresponding to the next die to be placed. Both manual and automatic swapping of die placement heads slows down the die placement process and manual die placement head swapping also poses a significant risk of contamination, since the system must be opened for the swapping operation. While automatic swapping of die placement heads is quicker than manual die placement head swapping, it still requires that the die placement head travel a significant distance within the system, resulting in the possible loss of placement accuracy due to tracking issues such as drift and an increase in cycle time due to time spent traversing this distance.
Today, with a wide variety of high density die package designs already available, with still more being developed every day, the requirements on die placement systems are increasing. Current generation dies require die bonding equipment that can deliver precision, versatility and speed beyond what is currently available to maximize productivity while minimizing defects resulting from faulty die placement, which, considering the effort that has already gone into creating and testing dies at this point in the production process, is an especially great concern. Future generations of dies are very likely to impose even higher requirements on die placement equipment.
What is needed, therefore, are techniques for improving the accuracy, precision and consistency of die placement and the application of die placement force while accelerating the die placement process, especially where multiple dies must be placed on a single substrate.